In the operation of many downhole tools employed in oilfield wellbores it is necessary to rotate the tools. For example, when a downhole tool encounters an obstruction which prevents further downward movement of the tubing string and tool, the tool can often be cleared from the obstruction by rotating it. Rotation is also performed for other purposes, such as aligning a tool with a bore in a packer seal.
Rotation of a downhole tool is normally carried out by rotating the tubing string to which the tool is attached. This is not a viable method, however, in all cases. For example, rotation of a downhole tool connected to the longer tubing string of a dual string arrangement is extremely difficult, and sometimes impossible, to carry out by this method due to the tendency of the strings to twist around each other when surface rotation is applied to both strings. Nor is rotation possible when the tool is connected to coil tubing or to a wireline. Prior to this invention, no practical solution to these problems has been available.
Another problem involves the retrieval of drill pipes which become stuck or lost when drilling a well. Conventionally, a threaded tool on the end of a tubing string is used to "fish" for the lost drilling sections. The tool is aligned with the upper end of the lost pipe, attached to it, then rotated in a direction to unthread the attached section from the other lost pipe sections. When left-hand torque is applied to the tubing string at the surface, there is considerable risk that the torque might loosen one of the threaded joints in the tubing string before the lost pipe section is loosened, resulting in lost time and expense in retrieving the separated tubing string as well as the lost pipe sections.
One suggested way of imparting rotary movement to a downhole tool for the purpose of screwing or unscrewing sections of a well casing or tubing string is by use of a so-called "Yankee screwdriver". Such structure utilizes a helical connection between two elements which imparts rotary motion to one of the elements upon relative linear motion between the elements. Normally, the tubing string is provided with means for gripping the casing in which it is located to prevent rotation of the string. Such an arrangement, however, involves a more complicated and expensive structure than is desirable and is not suited to a rapid retrieval operation.
It would be desirable to be able to impart rotational motion to a downhole tool in a simple, inexpensive manner without requiring rotation of the tubing string to which it is connected in order to carry out various operations, such as those mentioned above.